508 Heredity in Protozoa 



environmental factors, were observed by Raffel (65, 66) in a clone of 

 Paramecium aurelia. The new types did not revert to normal, even after 

 conjugation, and were believed to have arisen by gene mutation. The 

 origin of two unusual biotypes — differing from the parent stock in rarity 

 of conjugation, lower division-rate, higher mortality, and frequency of 

 morphological abnormalities — also has been reported by Sonneborn and 

 Lynch (92) in P. aurelia. 



Some of these hereditary changes in ciliates were attributed to endo- 

 mixis (5, 15, 92). Diller's (11) report of autogamy in P. aurelia was dis- 

 counted as a possible explanation on the grounds that his evidence for 

 autogamy was far from convincing and that extraordinary assumptions 

 would be necessary in relating autogamy to the appearance and subse- 

 quent disappearance of particular traits (92). The recent conclusion of 

 Sonneborn (81), that autogamy (and not endomixis) occurs in his strains 

 of P. aurelia, evidently leaves some of these intraracial changes in ciliates 

 unexplained for the present. The effects of selection described in Stylony- 

 chia pustulata cannot be ascribed to endomixis or autogamy because 

 neither process seems to have been observed in this speties. 



Certain morphological changes in ciliates have been interpreted as 

 mutations. An example is Hance's (19) race of Paramecium caudatum 

 with 2-7 extra contractile vacuoles, an abnormality inherited in fission 

 and vmaffected by selection or conjugation. Likewise, hereditary changes 

 in number of nuclei have been observed in P. bursaria (101). A trun- 

 cated type of P. aurelia has shown similar behavior, persisting through 

 more than 400 generations without being influenced by conjugation or 

 endomixis (9). MacDougall's (43) tetraploid mutant in Chilodonella 

 uncinatus also bred true. 



Environmentally induced changes 



A variety of changes may be induced by modification of environ- 

 mental conditions. Although some cases of acclimatization may represent 

 merely the selection of a resistant strain from a genetically mixed pop- 

 ulation, serologically distinct types evidently can arise within a pure line, 

 as in Trypanosoma brucei (67). 



Changes in resistance to chemical agents have been investigated in 

 parasitic and free-living species. Among the parasites, most of the work 

 has been done on trypanosomes- in which antigenic modifications, occur- 

 ring during an infection, are especially interesting. After inoculation of 

 a guinea pig, for example, with Trypanosoma rhodesiense, the flagellates 

 increase in number for a time. Suddenly, most of them are killed by a 

 newly developed antibody. The survivors continue to multiply, so that 



* Papers by Dobell (14) and Taliaferro (93) may be consulted for references to the 

 earlier literature. 



